1. Field of the Invention
The present invention relates to a radiation-curable vinyl chloride resin composition and to a method of manufacturing the same. More particularly, the present invention relates to a radiation-curable vinyl chloride resin composition having both good storage stability and curability, and to a method of manufacturing the same.
The present invention further relates to a vinyl chloride resin formed of the above composition, a magnetic recording medium comprising a radiation-cured layer formed of the above composition, and a storage stabilizer for radiation-curable vinyl chloride resin.
2. Discussion of the Background
In particulate magnetic recording media, binders play important roles in the dispersibility of magnetic particles, coating durability, electromagnetic characteristics, running durability, and the like. Accordingly, various research has been conducted on binders for magnetic recording media.
Conventionally, thermosetting resins and thermoplastic resins such as vinyl chloride resins, polyurethane resins, polyester resins, and acrylic resins have been widely employed as binders in magnetic recording media. In contrast, in recent years, the use of radiation-curable resins incorporating radiation-curable functional groups as binders for magnetic recording media has been proposed to obtain tougher coatings with good productivity. For example, Japanese Unexamined Patent Publication (KOKAI) No. 2004-352804 or English language family member US2004/0241497A1 and U.S. Pat. No. 7,001,955, Japanese Unexamined Patent Publication (KOKAI) No. 2005-8866, and Japanese Patent No. 3,125,947, which are expressly incorporated herein by reference in their entirety, propose the use of radiation-curable vinyl chloride resin as a magnetic recording medium-use binder.
Radiation-curable resins are generally synthesized either by using a monomer having a radiation-curable functional group to conduct a polymerization reaction, or by reacting a compound having a radiation-curable functional group with a polymer to introduce a radiation-curable functional group into the side chain of the polymer. These reactions are normally conducted in the presence of a polymerization-inhibiting agent to prevent the radiation-curable functional group from reacting. For example, the above publications describe conducting the synthesis reaction of a radiation-curable vinyl chloride resin in the presence of 2,6-tert-butyl-4-methylphenol (BHT) or hydroquinone.
In the course of the large-scale production of particulate magnetic recording media, the coating liquids are sometimes stored for longer periods of half a year or more, for example. However, vinyl chloride binders are generally of poor stability. In particular, when employing a radiation-curable vinyl chloride resin, the stability of the coating liquid may decrease markedly. This has been attributed to change in the molecular weight due to the reaction of radiation-curable functional groups during storage. However, when the above polymerization inhibiting agent is increased to inhibit reaction of the radiation-curable functional groups during storage, the curability during irradiation may decrease, making it difficult to obtain a tough coating.
No means of achieving both long-term storage stability in a radiation-curable vinyl chloride resin and curability when irradiated with radiation has been discovered thus far.